A lithium ion secondary battery has less weight and more capacity as compared with a conventional lead secondary battery, a nickel-cadmium secondary battery and so forth and hence has been widely used as a power source for electronic devices such as a mobile phone and a notebook type personal computer. It has recently been used also as a battery for an electric vehicle, a plug-in hybrid car, a pedelec and so forth.
The lithium ion secondary battery is basically composed of a cathode, an anode, an electrolyte, and a separator.
For the anode, in general, carbon, lithium titanate or the like which allows metallic lithium or lithium ion intercalation/deintercalation is used. For the electrolyte, lithium salt and an organic solvent or an ionic liquid capable of dissolving the lithium salt are used. The separator is placed between the cathode and the anode so as to keep electrical insulation therebetween and has pores to allow the electrolyte to pass through. For the separator, porous organic resin, glass fiber or the like is used.
The cathode is basically composed of an active material which allows lithium ion intercalation/deintercalation, an electrically conductive auxiliary which ensures an electrical conduction path (electron conduction path) to a current collector, and a binder which binds the active material and the electrically conductive auxiliary. As the electrically conductive auxiliary, a carbon material such as acetylene black, carbon black or graphite is used. As the active material of the cathode material, a metal oxide composed of lithium and a transition metal (s), such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2 or LiMn2O4, is used. Other examples are LiMPO4, derivatives obtained from this lithium metal phosphate as the basic structure by element substitution or compositional change, Li2MSiO4, derivatives obtained from this lithium metal silicate as the basic structure by element substitution or compositional change, LiMBO3, and derivatives obtained from this lithium metal borate as the basic structure by element substitution or compositional change. M mainly contains a transition metal element (s) having a variable valency, such as Fe, Mn, Ni, and Co.
This kind of metal oxide generally has low electron conductivity, and hence, in the cathode which uses the metal oxide as the active material, the metal oxide is mixed with the electrically conductive auxiliary as described above. Efforts have been made to further improve the electron conductivity inside the cathode by coating the surface of the metal oxide as the active material with carbon or by making carbon particles, carbon fiber or the like adhere to the surface of the metal oxide, in addition to mixing the metal oxide with the electrically conductive auxiliary. (Refer to Patent Literatures 1 to 6 and Non-Patent Literature 1, for example.)
In particular, with respect to the metal oxide having significantly low electron conductivity, even if the cathode is configured by making the electrically conductive auxiliary and the metal oxide coexist therein, it is not enough to obtain excellent battery characteristics. Hence, in order to use such metal oxide, the surface of the metal oxide is coated with carbon.
Among the oxides described above, lithium iron silicate Li2FeSiO4, lithium manganese silicate Li2MnSiO4, and derivatives obtained from these as the basic structure by element substitution or compositional change each contain two lithium ions in one composition formula, so that high capacity can be expected theoretically. (Refer to Patent Literatures 7 to 11 and Non-Patent Literature 2.) Since each of these oxides has significantly low electron conductivity, an attempt to coat the oxide grains with carbon has been made in addition to mixing the oxide with the electrically conductive auxiliary in an electrode. (Refer to Non-Patent Literatures 3 to 5.)